It is well known that the propagation velocity of ultrasonic waves through a sample gas is presented by a function of the concentration and the temperature of the sample gas. The velocity of ultrasonic waves C(m/sec) propagating through a sample gas is presented by following equation (1) with mean molecular weight M and the temperature T(K).C=(κRT/M)1/2  (1)Where;    κ: ratio of molecular specific heat at constant volume and molecular specific heat at constant pressure    R: universal gas constant
Therefore measuring the velocity of ultrasonic waves C(m/sec) propagating through a sample gas and the temperature T(K) of the sample gas will provide the mean molecular weight M of the sample gas through a calculation. For example, the mean molecular weight M of a sample gas containing an oxygen-nitrogen gas mixture of a mixture ratio P:(1−P) (0≦P≦1) will be calculated by M=MO2P+MN2(1−P), where MO2: Molecular Weight of oxygen and MN2: Molecular Weight of nitrogen. Therefore, the oxygen concentration P will be obtained through a calculation on the basis of the measurement of mean molecular weight M. When the sample gas is an oxygen-nitrogen mixture, κ=1.4 is reasonable over a wide range of the oxygen-nitrogen mixture ratio.
When the velocity of ultrasonic waves propagating through a sample gas is C(m/sec) and the flow velocity of the sample gas is V(m/sec), the velocity of ultrasonic waves V1(m/sec) propagating in the forward direction relative to the sample gas flow is V1=C+V, and the velocity of ultrasonic waves V2(m/sec) propagating in the backward direction relative to the sample gas flow is V2=C+V. Therefore, the velocity of the sample gas flow V(m/sec) is calculated by following equation (2).V=(V1−V2)/2  (2)
The flow rate (m3/sec) of the sample gas will be obtained by multiplying this by the sectional area (m2) of the conduit through which the sample gas flows.
Methods and apparatuses for measuring the concentration of a certain gas or the flow velocity of a sample gas, by using the above principle, on the basis of the propagation velocity or the propagation time of ultrasonic waves through the sample gas have been developed. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-213877 describes an apparatus for measuring the concentration and the flow rate of a sample gas by measuring the propagation time of ultrasonic waves propagating between two ultrasonic transducers opposingly disposed in a conduit through which the sample gas flows. Further, Japanese Unexamined Patent Publications (Kokai) No. 7-209265 and No. 8-233718 describe an apparatus for measuring the concentration of a certain gas contained in a sample gas by measuring the propagation velocity or propagation time of ultrasonic waves propagating through a volume with a reflecting type apparatus including a ultrasonic transducer and an opposingly disposed reflector.
In such a method and an apparatus for measuring the concentration and the flow rate by using the propagation velocity of the ultrasonic waves, it is necessary to accurately determine the propagation length of the ultrasonic waves, that is the distance between the transducers or between the transducer and the reflector, and the inner diameter of the conduit. However, the propagation length and the inner diameter of a conduit are adversely affected by the changes in the size of the conduit due to the changes in the temperature of the sample gas. Further, the propagation length of ultrasonic waves and the inner diameter of a conduit are also affected by the accuracies in machining or assembling the conduit, assembling the ultrasonic transducer and the reflector, and machining the ultrasonic transducer. Therefore, it is difficult to obtain the propagation length of ultrasonic waves and the inner diameter of a conduit accurately, which reduces the measurement accuracy.
Above described Japanese Unexamined Patent Publications (Kokai) No. 6-213877 and No. 8-233718 describe a temperature correction factor introduced to improve the temperature characteristics of the concentration measurement results. Further, there is a method in which the relations between the temperature, the propagation velocity of ultrasonic waves and the concentration are stored in a memory device as a table. However, in order to obtain such a temperature correction factor or table, a sample gas must be supplied to the device at various different temperatures to previously obtain the temperature characteristics of the apparatus. Therefore, a large amount of effort is required.
Further, a method for minimizing the temperature characteristics of the measurement results has been proposed in which whole of an apparatus is disposed under a temperature control for the measurement at a constant temperature. However, in this method, there is a problem that it is difficult to accurately control the temperature of the apparatus, in particular the conduit in addition to the necessity of a separate facility for conducting the temperature control.